1. Field of the Invention
The present invention relates to a step variable gain amplifier for amplifying an amplification rate of a voltage component of a signal received from an antenna according to each of gain modes by using two transconductance amplifiers, an attenuator, and at least one switching device.
2. Description of the Related Art
Recently, the line of products are being developed that can perform multifunction by transmitting and receiving specific information in either a mobile communication system, or a digital TV.
However, the amplitude of a transmission signal generated during the process is distorted according to a transmission device, a transmission condition, or channel characteristics, and thus such a distorted signal fails to meet linear characteristics of a system.
A method for reducing distortion characteristics of a signal is to adjust the amplitude of an initially transmitted signal by varying the amplitude of the signal again so that optimal signal processing can be implemented. The amplitude of this signal can be adjusted through a variable gain amplifier.
That is, the variable gain amplifier positioned at the front stage of a wireless communication device and a broadcasting receiver is called a low-noise amplifier. A noise figure of the low-noise amplifier is very important because it is a factor for determining noise figures of all receivers.
The amplitude of received communication or broadcasting signals ranges from a very small signal to a large signal. When a large signal is received, the low-noise amplifier needs to reduce a gain and attenuate the received signal in order to ensure linearity of the whole system.
There is a method for controlling a gain in multi-steps in front stage of an amplifier as a method to attenuate the signal.
FIG. 1 is a circuit diagram of a discrete-step gain switch amplifier using a conventional switch.
As shown in FIG. 1, the discrete-step gain switch amplifier circuit using the conventional switch includes a switch attenuation unit 110, and a fixed gain unit 120. The switch attenuation unit 110 includes switches SW11 to SW14, resistors R11 to R16, and an output terminal.
The fixed gain unit 120 includes an amplifier 121, resistors Rf1 and Rf2, and an output terminal, and an output terminal of the switch attenuation unit 110 is interconnected to an input terminal of the fixed gain unit 120.
In the case where all switches SW11 to SW14 of the switch attenuation unit 110 are switched off, a circuit fails to be configured even if an input signal is applied to the switch attenuation unit 110. Therefore, no amplification of signal is made in the amplifier 121 of the fixed gain unit 120, resulting in no output.
In the case where the switch SW11 of the switch attenuation unit 110 is switched on for configuration of the circuit, an input signal is applied to a first input terminal of the fixed gain unit 120, and an output terminal of the switch attenuation unit 110 is connected to a second input terminal of the fixed gain unit 120 by composite resistance of resistors R11 to R16.
By turning on/off the switches SW11 to SW14 of the switch attenuation unit 110, the gain of the amplifier 121 of the fixed gain unit 120 can be changed.
In a low gain, an effect of the noise figure is weak since the switch attenuation unit 110 is arranged at the input terminal of the fixed gain unit 120. However, in a high gain, amplification and conversion operation of the amplifier 121 of the fixed gain unit 120 cause switching devices to be driven, so a switching noise and a thermal noise resulting from the switching devices are produced within the amplifier 121. Therefore, the noises are added to a signal and accordingly, a noise figure is deteriorated.
FIG. 2 is a circuit diagram of a variable gain low noise amplifier in which the conventional amplifiers are connected in parallel to one another.
As shown in FIG. 2, the circuit of the variable gain low noise amplifier in which the conventional amplifiers are connected in parallel to one another includes first to fourth selection units 201 to 204, a power supply unit, and input and output terminals.
The first selection unit 201 includes capacitors C11 and C12, transistors M11 and M12, resistors R21 and R22 for driving, and the second selection unit 202 includes capacitors C21, C22, and C23, transistors M21 and M22, and resistors R23 and R24 for driving.
Further, the third selection unit 203 includes capacitors C31, C32, and C33, transistors M31 and M32, and resistors R25 and R26 for driving, and the fourth selection unit 204 includes capacitors C41, C42, and C43, transistors M41 and M42, and resistors R27 and R28 for driving.
When a current I1 is applied to the base of the transistor M12 of the first selection unit 201, a current between the collector and the emitter of the transistor M12 is amplified and flows, and a current between the collector and the emitter of the transistor M11 flows. Thus, a gain of the current is determined by the capacitor C12.
When a current I2 is applied to the base of the transistor M22 of the second selection unit 202, the transistor M22 is turned on, and a gain is determined by a ratio of two capacitors C22 and C23 connected to the transistor M22 as a current between the collector and the emitter of the transistor M22 flows.
An operation principle of the third or the fourth selection units 203 or 204 is also the same as the above-described method.
However, parallel connection of amplification devices and applying of a current for gain selection results in deterioration of a bandwidth. Further, since a current is applied for a variable gain, the total consumption power is increased.